Sobre la diversidad conceptual y epistémica del Espacio y el Tiempo

IBD is characterised by immunoregulatory effects in the mucosa, which appear to be associated with microbial exposure. Normally the relationship between commensal bacteria and the host is symbiotic as it is has been hypothesised that exposure to commensal bacteria in healthy individuals down-regulates inflammatory genes which leads to the inhibition of the NF-ϰB pathway and thus the immune response to the constant mirage of microbes and food antigens is inhibited (Donnenberg, 2000, Neish et al., 2000, Abreu, 2002, Sands, 2004). However in IBD this tolerance is lost and constant exposure to microbes and food antigens now triggers an inflammatory response subsequently causing a chronic destructive immune response. This has been demonstrated in mouse models by showing that colitis will not develop in

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mutated strains of mice maintained in a germ free environments, conversely when a single commensal or mixed bacteria load is introduced, this results in rapid mucosal inflammation (Rath et al., 2001). IBD is also associated with increased permeability of the gut epithelial lining resulting in continuous stimulation of the mucosal immune system. In animal models it appears the most severe inflammation develops in the location of the permeability defect (Hanauer, 2006).

T helper cells are mediators of inflammation producing differential cytokine production that can drive one of several different inflammatory pathways (Hendrickson et al., 2002). Evidence suggests that IBD may follow 1 of 2 pathways: an excessive T helper 1 phenotype, which is associated with CD or an excessive T helper 2 phenotype which is linked to UC. It is important to note that this dichotomy is in some individuals, is oversimplified and it is possible to have a combination of Th1 and Th2 pathologies in IBD patients (Bamias et al., 2005).

Over production of IL-12, a macrophage-derived cytokine shifts the immune response in favour of a T helper 1 pathway. This response is characterised by increased production of interferon-γ, Tumour necrosis factor (TNF)-α, interleukin (IL) - 1β, IL-2 and IL-6 and results in a self-sustaining cycle of activation (Abreu, 2002, Hendrickson et al., 2002, Bouma and Strober, 2003, Fuss, 2003). An excessive T helper 2 response is associated with increased secretion of IL-4, IL-5, IL-10, and IL- 13. T helper 2 cells also support the humoral immune response (Hendrickson et al., 2002). Mucosal inflammation may also result from a defect in the mature T cells, T helper 1 cells, suppressor cells that produce transforming growth factor (TGF)-β, IL- 10, and other immune-inhibitory cytokines. Such a defect would accelerate a loss of tolerance to ordinary antigens in the mucosal microflora, resulting in proliferation and production of inflammatory cytokines. This theory is supported by experimental studies where IL-10 deficient mice develop colitis and delivery of TGF-β or IL-10 ameliorates the colitis (Abreu, 2002).

T helper 17 (Th17) cells have also been linked to IBD pathophysiology particularly CD. Th17 cells are a group of IL-17 producing T cells linked to autoimmunity (Brand, 2009). It was widely believed that chronic intestinal inflammation that is characteristic of IBD is the consequence of pathogenic Th1 CD4 cell responses against the luminal flora, which in turn is driven by pro-inflammatory cytokines such as IL-12 and TNF

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(Sartor, 2006, Shih and Targan, 2008, Yadav and Liu, 2009). However recently evidence of high number of CD4 Th17 cells found in colonic lamina propria in mice and further analysis confirmed that commensal gut flora contribute to the expansion of these CD4 Th17 cells, leading to mucosal inflammation (Niess et al., 2008, Ivanov et al., 2009). IL-17 mRNA has been found to be highly expressed in inflamed mucosa from both UC and CD patients and it has also been shown that transcripts for Th17 related cytokines were increased in both UC and CD patients (Fujino et al., 2003). The significance of Th17 immunity in UC is further supported with studies showing that recombinant IL-23 actually enhanced IL-17 production by lamina propria CD4 T cells in UC, however had a lesser effect on CD4 T cells in CD. This could be due to the Th1 pathway which has been reported to antagonise the Th17 pathway via various mechanisms (Liu et al., 2009).

1.2 Current IBD Therapies

CD and UC have similar treatment regimes. Yet each treatment regime is specific to each individual as the physician sees fit. Currently there are five drug categories that may be prescribed to patients with IBD: anti-inflammatory, immunosuppressive, antibiotics, probiotics and biological therapy (Hanauer and Baert, 1994). Anti- inflammatory drugs are the most commonly prescribed treatments for both UC and CD. These include the drug mesalazine and a group of drugs known as corticosteroids (prednisolone, methylprednisolone, butesonide) (Pithadia and Jain, 2011). Mesalazine is only effective for mild-moderate sufferers of UC and shows no significant effect on CD (Camma et al., 1997). Few side-effects have been reported, however there is a risk of renal dysfunction. Of the patients prescribed corticosteroids, approximately one third failed to respond to this treatment in both UC and CD. This result could be explained by the differential expression of the glucocorticoid receptor (GR) promotor usage in T cells (Purton et al., 2004); not all T cells produce equivalent GR transcript and thus they may possess different susceptibility to glucocorticoid induced cell death, potentially accounting for why corticosteroid administration is ineffective.

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IBD sufferers on corticosteroids also have the risk of several side-effects such as weight gain, thinning of the skin, glaucoma, diabetes and osteoporosis (Pithadia and Jain, 2011). Immunosuppressive drugs are also a commonly prescribed treatment for both UC and CD. This is due to the vast amount of evidence in support of an altered immune system playing a large role in the pathogenesis of IBD. General drug treatments that are used in IBD are azathioprine, methotrexate, cyclosporine, 6- mercaptopurine and tacrolimus (Pearson et al., 1995, Aberra et al., 2003). Azathioprine and 6-mercaptopurine both fail to precipitate a response to one third of the recipients and as a collective, this group of drugs can have severe side-effects such as renal dysfunction, hepatic injury, tremors, pancreatitis, hypertension, nausea and diarrhoea (Pithadia and Jain, 2011). The third category of prescribed drugs for IBD sufferers is antibiotics. Some common courses of antibiotics recommended are metronidazole, clarithromycin and ornidazole (Sutherland et al., 1991, Arnold et al., 2002). Antibiotics are only necessary if bacterial infection or a gastrointestinal tract (GIT) micro-flora imbalance is diagnosed (Sartor, 1995).

Probiotics are also used in the treatment of IBD; the main side-effects reported for this course of treatment are flatulence and bloating causing severe discomfort. Patients that are on an immunosuppressive therapy should not be prescribed probiotics due to a risk of causing a micro-flora imbalance (Pithadia and Jain, 2011). Probiotics are not commonly prescribed as optimisation of the probiotic; dose and patient population have not been conducted. Over the past decade IBD has emerged as one of the most studied conditions linked to gut microbiota (Swidsinski et al., 2002, Sartor, 2008). With some studies theorising that IBD pathogenesis may result from dysregulation of the mucosal immune system driving a pathogenic immune response against the commensal gut flora (Strober et al., 2007). Short-term treatment with enterically-coated antibodies dramatically reduced intestinal inflammation and has been demonstrated to have some efficacy in IBD (Casellas et al., 1998, Sartor, 2004). Studies have also consistently reported a decrease in alpha- diversity in IBD, a measure of the total number of species in a community. Reduced alpha-diversity in faecal microbiome has been shown in CD, specifically within the Firmicutes phylum (Kang et al., 2010b) and interestingly a reduced diversity has been shown in inflamed tissue versus non-inflamed tissue within the same patient. A major study recently analysed over 1000 patients-naïve samples, which were

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collected from multiple concurrent gastrointestinal locations, from patients representing a variety of disease phenotypes with respect to location, severity, and behaviour. This study indicated that assessing the rectal mucosa-associated microbiome offers a potential for convenient and early diagnosis of CD (Gevers et al., 2014).

The last category of drugs prescribed for IBD patients is biologics. The most common biologic on the market is infliximab, a tumour necrosis factor alpha (TNF-) blocker (Targan et al., 1997). TNF- is a cytokine primarily involved in inflammation and if the receptor of TNF- can be blocked, inflammation will then decrease. Side- effects of infliximab can include heart failure, malignancies, autoimmunity and opportunistic infection (Triantafillidis et al., 2011). There are also many new biologic therapies in clinical trials that are of interest to IBD research scientists. The challenge for these researchers are to find specific receptors and corresponding blockers for key cytokines involved in the inflammatory process in order to ameliorate inflammation (Cohen, 2010). The issue with these biologic therapies is that the majority of the IBD population are not-responsive as the treatment does not prevent relapsing of the disease (Cohen, 2010).

1.3 Mesenchymal Stem Cells (MSCs)

One emerging therapy for IBD is mesenchymal stem cells (MSCs). Cells from bone marrow are aspirated and cultured in plastic flasks, hematopoietic cells and hematopoietic stem cells (HSCs) do not adhere to plastic.Once removed the remaining adherent cells were originally called colony-forming unit fibroblasts now referred to as MSCs. Like HSCs, MSCs are rare in bone marrow, representing only 1 in 10,000 nucleated cells. MSCs are multi potent bone marrow cells able to differentiate in cells from mesenchymal origin such as adipose cells, bone cells, muscle cells and cartilage cells. MSCs provide the support for the growth and differentiation of hematopoietic progenitor cells in bone marrow microenvironments (Noort et al., 2002, Dalal et al., 2012). MSCs have been observed inhibiting T cell proliferation in vitro in 2002- 2003 by three independent investigators which opened the door for the use of MSCs for autoimmune disorders, first in animal models and then in humans (Bartholomew et al., 2002, Di Nicola et al., 2002, Le Blanc et al.,

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2003, Dalal et al., 2012). In co-culture experiments with allogenic lymphocytes, MSCs do not induce lymphocyte proliferation, interferon-γ production, or upregulation of activation markers. Despite this ex vivo property, survival of infused allogenic MSCs in immunocompetent mice was estimated to be less than 40 days and when mice were rechallenged, survival of infused MSCs was less than 5 days. There was also evidence of immune memory induction by MSCs suggesting that MSCs cannot completely evade the immune response and are eventually rejected (Bartholomew et al., 2002, Zangi et al., 2009, Dalal et al., 2012). MSCs suppress proliferation of activated lymphocytes in vitro in dose-dependent, non-human leukocyte antigen-restricted manner. In a baboon skin-graft model, investigators showed that infusion of ex-vivo expanded donor-derived or third party cells prolonged the time to rejection of histoincompatible skin grafts. Furthermore, infused cells improve the outcome of acute renal, neural, and lung injury, possibly by promoting a shift from production of pro-inflammatory to anti-inflammatory cytokines at the site of injury (Ortiz et al., 2003, Tögel et al., 2005, Zappia et al., 2005).

Once administered MSCs can migrate through chemotaxis towards the site of inflammation, specifically targeting pathological manifestations. After homing to the site of inflammation, MSC can facilitate tissue regeneration through the secretion of pro-angiogenic and trophic factors, which have been shown to promote endogenous repair mechanisms (Stavely et al., 2014). MSCs appear to be immunomodulatory and secrete anti-inflammatory factors suppressing the immune response and inflammation.